British intelligence has identified 2,000 people with links to international terrorism.
In his first public speech, reports the UK's Independent , the new head of British's domestic intelligence service, Jonathan Evans, warned that al Qaeda is grooming British teenagers for terror attacks against the United Kingdom.
"As I speak, terrorists are methodically and intentionally targeting young people and children in this country," he said.
"They are radicalising, indoctrinating and grooming young, vulnerable people to carry out acts of terrorism. This year, we have seen individuals as young as 15 and 16 implicated in terrorist-related activity."
Mr Evans said the terror plots were being directed from a widening range of countries.
While in the past much of the planning had come from al-Qa'ida's "core leadership" in the tribal areas of Pakistan, there were now signs that al-Qa'ida in Iraq was seeking to promote attacks outside that country while there was terrorist training and planning in Somalia directed against the UK.
Evans also said that the United Kingdom has seen an uptick in the number of people MI-5 has identified with links to terrorism, increasing from 1,600 last year to 2,000 at present.
In his speech before the Society of Editors Conference in Manchester, Evans also told how MI-5 is spreading throughout the country.
Mr Evans said MI5's focus was becoming increasingly regional, with eight offices across Great Britain, and around 25% of the staff expected to be working outside its London headquarters by 2011.
At the same time, a new Northern Ireland headquarters building will be opened shortly, with the capacity to contribute to MI5's overall counter-terrorism effort as well as particular issues relating to the province.